A suspension system for a vehicle typically includes the use of shock absorbers, shock absorbing struts and/or suspension springs to provide a cushioned ride for the driver and the passengers, if any, of the vehicle. A conventional shock absorber has a chamber filled with hydraulic oil in which a piston connected to a shaft is moved. The viscosity of the oil in the cylinder provides damping for the movement of the piston and shaft relative to the cylinder so that any vibration or bump to either the cylinder or the piston and shaft that result from sudden jolts to the vehicle is modulated.
A second type of shock absorber consists of gas shocks that use pressurized gases for damping the movement of the piston and shaft. Yet a third type of suspension system comprises a combination of both oil and gas. For this type of suspension system, there is provided an external reservoir for the oil that is separate from the shock absorber cylinder. In this reservoir, there is stored, in addition to the oil, a pressurized gas that provides a biasing force against the oil so that, when oil is needed in the cylinder, the oil in the reservoir is forced by the pressurized gas to fill up the cylinder. When under pressure in the cylinder, the excess oil is forced out of the cylinder and stored in the reservoir.
External oil reservoir suspension systems work well but are bulky requiring a large amount of space which may not be available for certain types of vehicles such as for example snowmobiles or race cars. Moreover, due to the need for an extra reservoir and the conduits necessary for connecting the reservoir to the shock absorber, the cost for such system is much higher and therefore could not be justified in most types of vehicles. Furthermore, due to the extra valves and controls that are required to regulate the flow of fluid between the shock absorber and the reservoir, not only is the cost high, the maintenance and reliability for such systems is greater than that required for a typical suspension system. Lastly, the chances of a breakdown for such suspension system are increased due to the additional components and connections required.